In 3GPP (3rd Generation Partnership Project) which is the international standardization organization for mobile communication, the standardization of LTE (Long Term Evolution) which is a 3.9-generation mobile communication system has been completed, and at present, the standardization of LTE-Advanced (LTE-A) which follows LTE is being advanced as a 4th generation mobile communication system. In LTE-A, as described in NPL 1, for the purposes of coverage expansion and capacity improvement, a Relay technique which relays a radio signal using a relay station (Relay Node) is being studied.
The Relay technique in LTE-A will be described simply with reference to FIG. 15. FIG. 15 shows a communication system using Relay. In FIG. 15, eNB represents a base station, RN represents a relay station, and UE represents a terminal. UE1 is a terminal which is connected to eNB, and UE2 is a terminal which is connected to RN.
In LTE-A, similarly to the eNB, a technique in which an individual cell ID is given to an RN is being studied. Accordingly, similarly to a cell (macrocell) which is formed by the eNB, the RN can form a single cell (Relay cell). In LTE-A, the Relay is called Type 1 Relay. An eNB is connected to a network through wired communication, and an RN is connected to the eNB through wireless communication. A communication link which connects between the RN and the eNB is called a backhaul link.
Meanwhile, a communication link which connects between the eNB and the UE and between the RN and the UE is called an access link. In a downlink, the RN receives a signal from the eNB in the backhaul link and transmits a signal to the UE2 in the access link of the RN. In an uplink, the RN receives a signal from the UE2 in the access link of the RN and transmits a signal to the eNB in the backhaul link.
In LTE-A, the Relay in which the backhaul link and the access link are stored in the same frequency band is called In-band Relay. In In-band Relay, if the RN performs transmission and reception at the same timing, since a transmission signal strays into a reception signal and interference occurs, the RN may not perform transmission and reception at the same timing. For this reason, in LTE-A, a Relay method in which the timing of the backhaul link and the access link of the RN are allocated in a subframe unit is being studied.
In LTE-A, as shown in FIG. 15, a system in which a Relay cell with a small area is arranged to overlap the area of a macrocell is being studied. Since the size of the area of the cell depends on the transmission power of the downlink this system, the RN performs transmission in the downlink at power lower than the transmission power in the downlink of the eNB. In this system, as one method of determining a connection cell of the UE, there is a method in which connection is made to a cell with the maximum downlink reception power in the UE.
In the uplink of the LTE, in order to reduce inter-cell interference, a method in which transmission is performed at the suppressed transmission power of the UE is selected. In LTE, the transmission power of a PUSCH (Physical Uplink Shared Channel) which is used for data transmission of the uplink is expressed by Expression (1) in NPL 2.PPCSC(i)=min{PMAX, 10 log10(M(i))+Po(j)·PL+ΔIF(i)+f(i)}  [Equation 1]
In Expression (1), Po(j) is a reception target level, and α(j) is a coefficient which compensates for distance attenuation (path-loss) PL between the eNB and the UE measured in the UE, and is a parameter which is notified from the eNB to the UE in a RRC (Radio Resource Control) message. The RRC message is control information of a higher level layer than a PDCCH (Physical Downlink Control Channel) which is control information of a physical layer. The eNB sets Po(j) to be lower or sets α(j) to a value smaller than 1, thereby controlling the transmission power of the UE to be low.
In Expression (1), f(i) is a cumulative value of a TPC (Transmit Power Control) command which controls variation in a reception level with movement of the UE or the like. The TPC command is a parameter which is notified from the eNB to the UE in the PDCCH. In a single notification of the TPC command, transmission power can be controlled to any value of [−1, 0, 1, 3] [dB]. PMAX represents the maximum transmission power at which the UE can perform transmission, M represents a transmission bandwidth, ΔTF(i) represents an offset relating to a transmission MCS (Modulation and Coding Scheme) set, represents a transmission subframe.
In the communication system using Relay shown in FIG. 15, a method of controlling uplink (Up Link; UL) transmission power is considered. In the communication system using Relay, even if a UE is closer to the RN than the eNB, since downlink reception power from the eNB is stronger than reception power from the RN due to a difference in downlink (Down Link; DL) transmission power between the eNB and the RN, there is a UE (UE1) which is connected to the eNB (hereinafter, a UE which is connected to the eNB is called a macrocell UE, and a UE which connected to the RN is called a Relay cell UE). In UE1, since the distance between eNB-UE1 is longer than the distance between RN-UE1, in regard to the path-loss (distance attenuation) depending on the distance, the path-loss between the eNB and the UE1 is larger than the path-loss between the RN and the UE1.
In the UE1, in order to obtain a desired uplink reception level in the eNB as a connection destination, it is necessary to transmit a signal at power which sufficiently compensates for the path-loss between eNB-UE1. The transmission power in this case is shown in FIG. 16. FIG. 16 is a diagram (first view) illustrating the transmission power of a plurality of terminals in an uplink. In FIG. 16, the vertical direction represents a power level, in which eNB and RN represent a reception level, and UE1 and UE2 represent a transmission level. The horizontal direction represents a distance. Similarly to FIG. 15, eNB represents a base station, RN represents a relay station, and UE represents a terminal. UE1 represents a terminal which is connected to eNB, and UE2 is a terminal which is connected to RN.
As shown in FIG. 16, since the distance between RN-UE2 is short, even if transmission is performed at a lower power than the UE1, the Relay cell UE (UE2) can obtain a desired reception level in the RN. However, in the RN, the reception power of a signal from the UE2 as a desired signal becomes higher than the reception power of a signal from the UE1 as an interference signal. If the macrocell and the Relay cell use the same band as the frequency band of the uplink, a signal from the macrocell UE (UE1) interferes with a signal from the Relay cell UE (UE2).
In regard to interference described with reference to FIG. 16, as studied in NPL 3, the macrocell UE (UE1) performs transmission at the suppressed transmission power, thereby suppressing interference with the Relay cell. The transmission power in this case is shown in FIG. 17. FIG. 17 is a diagram (second view) illustrating the transmission power of a plurality of terminals in an uplink In FIG. 17, the vertical direction represents a power level, eNB and RN represent a reception level, and UE1 and UE2 represent a transmission level. The horizontal direction represents a distance. Similarly to FIG. 15, eNB represents a base station, RN represents a relay station, and UE represents a terminal. UE1 is a terminal which is connected to eNB, and UE2 is a terminal which is connected to RN.